1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser device and a process for producing the same, and, particularly, relates to a so-called selective oxidation type surface emitting semiconductor laser device and a process for producing the same.
2. Description of the Related Art
A highly densified semiconductor laser array is demanded as a light source of light communication and an optical computer. In a semiconductor laser array, plural semiconductor laser devices are arranged at certain intervals, and the laser devices are controlled independently. A edge emitting semiconductor laser device is not suitable as a semiconductor laser array because it can only be one-dimensionally arranged on one substrate. On the other hand, a surface emitting semiconductor laser device is promising because it can be two-dimensionally arranged on one substrate to have an advantage in production of a matrix array of high precision and high density.
A vertical cavity surface emitting semiconductor laser device, as one of the surface emitting semiconductor laser device, comprises an active region comprising an active layer and a spacer layer, and a pair of distributed Bragg reflectors (DBR) sandwiching the active region, in which an oscillator is constituted by the DBR, which emits light in a normal direction with respect to the substrate. The surface emitting semiconductor laser device is characterized in that the emission angle is small, the longitudinal mode interval is large, and an array can be easily formed, in comparison with the facet radiation semiconductor laser device.
An example of the vertical resonance surface emitting semiconductor laser device is a so-called selective oxidation surface emitting semiconductor laser device, which is obtained by inserting Al.sub.x Ga.sub.1-x As (0.98.ltoreq.x.ltoreq.1) in the vicinity of the active region, and oxidizing the outer periphery of the inserted Al.sub.x Ga.sub.1-x As with steam to realize electric current confinement. One example thereof disclosed in Appl. Phys. Lett., vol. 65, No. 1, p. 97-99 (1994) has the structure shown in FIG. 4, in which a triple quantum well active layer comprising In.sub.0.2 Ga.sub.0.8 As is sandwiched by a DBR comprising GaAs/AlAs, provided that the p-type DBR comprises only a pair of GaAs/AlAs, and the GaAs layer is attached as the upper layer. In the production of the selective oxidation surface emitting semiconductor laser device, the p-type GaAs is first worked into a circular shape having a diameter of 30 or 60 .mu.m by using a photolithography process and a wet etching process. The exposed p-type AlAs layer is then subjected to a heat treatment in a furnace heated to 475.degree. C. for about 3 minutes. On the heat treatment, steam is obtained by bubbling nitrogen, as a carrier gas, in deionized water maintained at 95.degree. C., has already been introduced in the furnace. The exposed AlAs layer is gradually oxidized in the horizontal direction, and finally a region of from 2 to 8 .mu.m square is formed that remains not oxidized. The oxidized region becomes aluminum oxide, which substantially does not let an electric current pass, to realize electric current confinement, and a refractive index distribution is formed between the aluminum oxide thus formed and the AlAs remaining to lower the threshold electric current.
One example thereof disclosed in SPIE, vol. 2683, p. 114-122 (1996) has the structure shown in FIG. 5, in which a triple quantum well active region comprising InGaAs is sandwiched by a pair of semiconductor DBRs. Layers of Al.sub.0.98 Ga.sub.0.02 As are inserted on and below the active region, and, after forming a mesa structure by etching until both the Al.sub.0.98 Ga.sub.0.02 As layers are exposed, the Al.sub.0.98 Ga.sub.0.02 As layers are then oxidized by using steam. The literature describes that a life time of 2,500 hours or more could be obtained by inserting the Al.sub.0.98 Ga.sub.0.02 As layers in the vicinity of the active region.
However, in the case where Al.sub.0.98 Ga.sub.0.02 As having a high Al concentration is oxidized with steam by using nitrogen as a carrier gas, a slight fluctuation of the Ga concentration influences the oxidation rate as shown in Electronics Lett., vol. 30, No. 24, p. 2043-2044 (1994). For example, the oxidation rate of AlAs is five times that of Al.sub.0.98 Ga.sub.0.02 As. Because the size of the electric current confinement region obtained by oxidizing a part of the Al.sub.x Ga.sub.1-x As largely influences the threshold electric current and the horizontal mode stability of the surface emitting semiconductor laser device, the control of the oxidation rate, i.e., the control of the composition of the Al.sub.x Ga.sub.1-x As, is important in production of a device having uniform optical output characteristics with good reproducibility. However, techniques required in the composition control of the Al.sub.x Ga.sub.1-x As, for example, the flow rate control, is of a more advanced nature than that in the DBR and the active region.
In the case where AlAs (x=1) is used, the composition control becomes easier than the case of Al.sub.x Ga.sub.1-x As (x.noteq.1), and the uniformity and the reproducibility of the device are increased. However, as described in Appl, Phys. Lett., vol. 69, No. 10, p. 1385-1387 (1996), there is a tendency that the mesa structure is dropped off after sudden temperature rise after oxidation (which is considered to correspond to a heat treatment to form an ohmic contact of the upper electrode), and the life time of the surface emitting semiconductor laser device is as short as 100 hours or less.